1. Field of the Invention
The invention relates to an apparatus for detecting an abnormal air-fuel ratio variation between cylinders of a multi-cylinder internal combustion engine, and particularly relates to an apparatus and a method for detecting a relatively large air-fuel ratio variation between cylinders of a multi-cylinder internal combustion engine.
2. Description of Related Art
In general, in an internal combustion engine provided with an exhaust gas control system using a catalyst, in order to perform the purification of pollutants in exhaust gas by using the catalyst at high efficiency, the mixing ratio between air and fuel in an air-fuel mixture combusted in the internal combustion engine, i.e., air-fuel ratio control is essential. In order to perform the air-fuel ratio control, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine and feedback control is performed such that the air-fuel ratio detected by the air-fuel ratio sensor is caused to match a given target air-fuel ratio.
On the other hand, in a multi-cylinder internal combustion engine, the air-fuel ratio control is usually performed on all cylinders by using the same control amount, and hence, even when the air-fuel ratio control is executed, there are cases where the actual air-fuel ratio varies between the cylinders. When the degree of the variation is small, the variation can be compensated by the air-fuel ratio feedback control, and the pollutant in the exhaust gas can be purified by the catalyst so that the variation does not affect exhaust emission and does not present a problem.
However, for example, when the fuel injection system of a part of the cylinders fails and the air-fuel ratio variation between the cylinders is increased, the variation deteriorates the exhaust emission and presents a problem. It is desirable to detect such a large air-fuel ratio variation as to deteriorate the exhaust emission as an abnormality. In particular, in the case of a vehicular internal combustion engine, in order to prevent the running of a vehicle with the deteriorated exhaust emission beforehand, it is required to detect the abnormal air-fuel ratio variation between cylinders in an on-board state (on-board).
For example, WO 2011/030451 discloses an apparatus that detects an abnormal variation based on the fluctuation amount per unit time of the output of an air-fuel ratio sensor. In the apparatus, considering that the fluctuation amount changes according to an intake air amount, the fluctuation amount is corrected according to the intake air amount.
Incidentally, when the abnormal air-fuel ratio variation between cylinders is detected, it is conceivable that the presence or absence of the abnormal variation is determined by comparing a parameter correlated with the degree of fluctuation of the output of the air-fuel ratio sensor with a given determination value.
Subsequently, it is conceivable that, considering that the parameter changes according to the intake air amount, the determination value as the comparison target of the parameter is changed according to the intake air amount.
However, the degree of the fluctuation of the output of the air-fuel ratio is influenced not by the magnitude of the intake air amount detected in an intake passage but by the flow amount of the exhaust gas that has actually reached the air-fuel ratio sensor. In addition, the air-fuel ratio sensor detects the air-fuel ratio of the exhaust gas that has actually reached the air-fuel ratio sensor. Further, a time lag called a transport delay is present before the exhaust gas generated in a cylinder actually reaches the air-fuel ratio sensor.
Conventionally, these matters have not been adequately considered, and hence it has been difficult to properly set the determination value. As a result, there has been a possibility of an erroneous determination and it has been difficult to secure sufficient detection accuracy.